Electron injection apparatus



Oct. 11, 1949. .J- P. BLEWETT 2,484,549

ELECTRON INJECTION APPARATUS Filed July so. 1947 Inventor"; John F. Blewett,

His ttor'ney.

Patented Oct. 11, 1949 ELECTRON INJECTION APPARATUS iIolm P. Blewett, East Patchogue, N. Y., assignor to General Electric Company, a corporation of New York Application July 80, 1947, Serial No. 764,833

Claims. 1

The present invention relates to apparatusfor accelerating charged particles and in particular, comprises a cathode assembly which is located within an acceleration chamber for projecting a stream of charged particles (ordinarily electrons) into an orbital path in which desired acceleration is to occur.

Although the benefits of my invention are not restricted to a particular type or configuration of accelerator, the structural features of my invention will be described and its mode of operation will be explained in connection with magnetic accelerators such as described. for example, in Kerst U. 8. Patent 2,297,305, patented September 29, 1942, and in an article by Westendorp and Charlton entitled "A Million Volt Induction Electron Accelerator," in Journal of Applied Physics, volume 16, October 1945, page 581. My invention also is applicable to accelerators in which electrons are accelerated by synchronously varying electric oscillations as described in a copending application Ser. No. 639,462 of Pollock and Westendorp, filed on January 5, 1946, and assigned to the same assignee as the present application.

It is the object of my invention to increase the efliciency of electron injection into an accelerating field by providing a construction whereby undesired premature discharge of electrons upon the frame-work or housing of an electron injector, 0r gun, is avoided.

When electrons are injected into a guiding and accelerating magnetic field, they would be rejected by such field if special conditions did not exist. Even under favorable conditions. such as described in the above-mentioned Kerst patent, it has been found that a considerable portion of the electrons introduced into an accelerating field are lost before desired acceleration has occurred. Apparentl the lost electrons are discharged either upon the structure of the cathode or upon the wall of the accelerating chamber. It is a characteristic of magnetic accelerators, such as mentioned above, that electrons injected correctly near a wall of the accelerating chamber will undergo radial oscillations of slowly decreasing amplitude. Present injection techniques are such that the amplitude of oscillation does not decrease fast enough to permitthe greater part of the emitted electrons to clear the structure of the injection means on subsequent revolutions.

In the injection apparatus to be described herein the electrons are injected in such a fashion that even a slow decrease in amplitude of radial oscillation is suflicient to bring the greater part of the electron beam clear of the injection structure.

In accordance with my present invention, an electron injector is provided having electrostatic deflecting means and being structurally so related to the acceleration chamber that electrons emerge into the acceleration orbit from the injector in a direction which just grazes the orifice of the injector. The electrons are caused to travel in an orbital path having such radius, in a herein described embodiment of my invention, that the electrons in their revolutions need clear the injector structure only by a distance equivalent to the wall thickness of the housing to avoid being discharged thereon. Repeated revolutions of the electrons to be accelerated thus are carried out with greater economy of electron utilization until a desired electron voltage has been attained. The electrons are utilized when sufilciently accelerated by known means for any desired purpose, for example, to produce X-rays by striking a target.

My invention will be described in greater detail in connection with the accompanying drawing. In Fig. 1 is shown a vertical section of an electron accelerator to which an injector embodying my invention may be applied; Fig. 2 is a horizontal section thereof taken on lines 2-2 of Fig. 1, the wall of the acceleration chamber being in part broken awa to show the location of the electron injector; Figs. 3 and 4 are enlarged detail views of injector structure and Fig. 5 is a diagram of electric connections.

The accelerator structure shown in Figs. 1 and 2 comprises essentially a laminated rectangular magnet I having a central laminated magnetic core member 2 which is spaced at opposite ends from the main magnet structure by non-magnetic discs 3, 4. Between the oppositely spaced tapered circular pole. pieces 5, 6 is located a toroidal acceleration chamber 1, which ordinarily consists of glass. The chamber is evacuated and sealed off or sealed to a pumping system for continuous evacuation. Projecting into the chamber 1 is the electron injector 8, the input wires 9, In of which are led into a stem ll through well-understood sealing means which here are not shown in detail. The electron injector as shown, is located close to .the inner wall of greater radius of the chamber 1.

As best shown in Figs. 3 and 4, the injector or cathode structure comprises a somewhat rectangular box-shaped hollow housing constituted by the plates l3 and I3, and a thermionic filamentary cathode it which is partially surrounded by accuse an elongated trough-shaped metal channel II which functions as a focusing device and heat shield. As shownin Fig. 5, the cathode is and the channel is are electrically connected. Hence. the channel which is slightly tilted with respect to the axis of the housing (see Fig. 3) is electrically charged and ccnstricts the'einitted elect car.

into an elongated beam. Unless deflected, the electron beam would strike the housing near the opening between the plates is and it as best shown in Fig. 3. It is deflected instead as will be described. Heating current is supplied to the cathode filament by the conductors l6, II which lead to the sealed-in leads Ill. The housing is connected to the conductors I8, 18'. The latter are joined together and connected by a grounded conductor is to an intermediate terminal of thesecondary winding of a transformer 20. The negative terminal of this secondary winding is conwhich otherwise would strike the housing are attracted by the positive charge of the plate 23 and emerge as a beam from the opening substantially parallel to and grazing the plate i 3.

The primary winding of the transformer 20 is connected by the conductors 26, 21 and in circuit with a rectifier 28 and a resistor 29 to a source oi power as represented by the secondary of a transformer 30. The capacitor bank 3i connected in series with the conductor 26 becomes charged through the rectifier. When the accelerator is to receive an energizing pulse of energy, the conductors 26, 21 are short-circuited by the 1. An accelerator for electrons comprising the combination or an evacuated toroid providing a path for the acceleration of electrons, means for providing a time-varying magnetic field tor causing acceleration of electrons in said toroid, an electron source, means for projecting electrons ircm scarce into said field and an e ectrcde located adjacent the emergence of said electrons for guiding said electrons into a direction substantlallyparailcl to the orbital path or said electrons in said toroid.

2. An accelerator for electrons comprising the combination or an evacuated toroid in which electrons may be accelerated, means for providing a time-varying magnetic field tor acc lerating electrons in said toroid, means in said roid for injecting electrons. said means comprising a housin: having an opening adjacent the desired orbit of electrons to be accelerated, a thermionic filathyratron tube 32 (by means not shown but well understood), thereby discharging the capacitor bank through the primary of the transformer 20. The main power supply for primary windings 33, 33 of the accelerator, Fig. 1, and the tuned capacitor circuit connected to the secondary windings ,35. 38 have not been shown as they are wellknown in the art, see for example, Journal of Applied Physics, vol. 16, pages 581-593 containing an,

article by Westendorp and Charlton. The pulse circuit of Fig. 5 is correlated with the power circuit by means also well understood and as described, for example, in Westendorp U. S. Patent 2,394,071.

As a consequence of my invention, the electrons to be accelerated are injected into the predetermined orbital path at a grazing angle to and avoiding the electron gun housing. As'the diameter of their orbit of revolution becomes slightly contracted by the accelerating field they miss striking the cathode housing intheir first as well as subsequent revolutions. Hence with a given cathode emissivity, the current output of the accelerator is increased materially as a consequence of my invention.

What I claim as new and desire to secureby Letters Patent of the United States is:

ment, a focusing device therefor, and a guide electrode adjacent said opening, and an energizing circuit connected to said housing, to said filament and to said guide electrode to maintain said housing at ground potential and said filament and guide electrode respectively at negative potential and positive potential with respect to said houss.

3. An electron injection apparatus which is adapted for application to an electron accelerator, said apparatus comprising the combination of a housing having an elongated opening, an elongated thermionic cathode in said housing, an electrostatic focusing shield whereby electrons emitted by said cathode are compressed into a restricted beam, said focusing device being oriented to'direct said beam uponsaid housing adjacent said opening, an electrode adjacent said housing causing said beam to be attracted away from said housing and to emerge through said opening in a direction grazing said housing.

4. A cathode structure which is adapted to inject electrons into a toroidal acceleration chamber of an accelerator, said structure comprising the combination 0! a thermionic electron source, a surrounding housing having an opening for the emergence of a beam of electrons and electrostatic means adjacent said opening for causing said beam to emerge at a grazing angle to said housing.

5. An electron injector assembly for magnetic accelerator application comprising the combination 0! a housing having an opening, a-thermionic cathode located therein, a focusing device partially surrounding said cathode and being tilted with respect to the'longitudinal axis of said housing passing through said opening and a plate-shaped guiding electrode located near the said opening for causing electrons emitted by said cathode to emerge from said opening.

JOHN P. BLEWE'IT.

REFERENCES CITED UNITED STATES PATENTS Name Date Kerst Feb. 5, 1946 Number 

